The EPUB® specification is a distribution and interchange format standard for digital publications and documents. EPUB® defines a way of representing, packaging and encoding structured and semantically enhanced Web content, including XHTML, CSS, SVG, images, and other resources, for distribution in a single-file format. EPUB® allows publishers to produce and send a single digital publication file (an ePub file) through distribution and offers consumers interoperability between software/hardware for unencrypted, reflowable digital books and other publications. EPUB® was initially standardized in 2007 as a successor format to the Open eBook Publication Structure or “OEB”, which was originally developed in 1999.
Two important components of the EPUB® specification are the Open Packaging Format (OPF) and the OEBPS Container Format (OCF). The OPF specification defines the mechanism by which the various components of an electronic publication are tied together and provides additional structure and semantics to the electronic publication. Specifically, OPF describes and references all components of the electronic publication (e.g. markup files, images, navigation structures), provides publication-level metadata, specifies the linear reading-order of the publication, provides fallback information to use when unsupported extensions are employed and provides a mechanism to specify a declarative global navigation structure (the NCX).
The OCF specification describes a general-purpose container technology in the context of encapsulating ePub publications and optional alternate renditions thereof. As a general container format, OCF collects a related set of files into a single-file container. OCF can be used to collect files in various document formats and for classes of applications. The single-file container enables easy transport of, management of, and random access to, the collection.
OCF defines the rules for how to represent an abstract collection of files (the “abstract container”) into physical representation within a ZIP archive (the “physical container”). OCF is the required single-file container technology for ePub publications. During the preparation steps in producing an electronic publication, OCF is used as the single-file format when exchanging in-progress publications between different individuals and/or different organizations. When providing an electronic publication from publisher or conversion house (Content Provider) to the distribution or sales channel, OCF is the recommended single-file format to be used as the transport format. When delivering the final publication to an ePub reading system or end-user, OCF is the required format for the single-file container that holds all of the assets that make up the publication.